EVAL-AD421EB

REV. A

–7–

EMI/EMC CONSIDERATIONS

On the AD421 board care was taken with the layout, grounding

and decoupling to reduce any effects that electromagnetic

disturbances might have on the AD421. Two ferrites, L1 and L2

are included on the AD421 board to provide an extra level of

security against EMI disturbances that come in on the loop(+)

and loop(-) lines. EMC susceptibility tests were carried out on

the AD421 board to check the level of performance obtained

from this board when subjected to various electromagnetic

disturbances. The aim of the susceptibility tests is to ensure that

the product has an adequate level of intrinsic immunity to

electromagnetic disturbance to enable it to operate as intended.

The EMI/EMC directive 89/336/EEC only applies to products

or systems and not to individual components. The AD421

evaluation board acts as a system in this test procedure. The

immunity tests performed included conducted immunity and

radiated immunity. Conducted immunity is a fast transient burst

test where fast transients are coupled onto the 4-20ma loop

twisted pair cable using a 1m capacitive clamp. Radiated

immunity testing involves subjecting the board under test to

various electric fields with frequencies varying from 30MHz to

1GHz.

Radiated Immunity (IEC 1000-4-3)

This set of experiments looked at the performance of the AD421

when subjected to various electric fields. Class B approval for

use in residential/commercial light industrial environments

requires the system to remain functional in a field of 3V/m from

30MHz to 1GHZ. Class A approval for use in industrial

environments requires the system to remain functional in a field

of 10V/m. This test is carried out in a stripline cell which is

essentially two metal plates placed in parallel, the AD421 board

is placed between these plates where it is subjected to an electric

field applied to the plates. The results obtained from this test

with the AD421 in its normal operating mode with Vcc =5V and

controlling the current in the loop to 10mA are as follows.

Board in Horizontal Orientation:

Field Strength Results Comments

3V/m

PASS

Range from 30MHz to 1GHz

5V/m

PASS

Range from 30MHz to 1GHz

10V/m

PASS

Range from 30MHz to 1GHz

This is class 1 performance, i.e. normal operation maintained

during application on the field.

Board in Vertical Orientation:

Field Strength Results Comments.

3V/m

PASS

Range from 30MHz to 1GHz

5V/m

PASS

Range from 30MHz to 1GHz

10V/m

FAIL

Device resets to 4mA at 860MHz

This is class 1 performance, ie normal operation maintained

during application of the fields of 3V/m and 5V/m. The classifi-

cation is class 3 at 10V/m as user intervention is required to get

the AD421 back to its programmed conditions.

Conducted Immunity (IEC 1000-4-4 Fast Transient Burst

Test)

This set of experiments involved the coupling of fast transients

onto the current loop unshielded twisted pair cable through a

1m capacitive clamp. The fast transient burst is specified to

have a rise time of 5ns and a duration of 50ns and is supplied

from a 50

Ω source. Bursts of 15ms duration of these pulses at a

repetition rate of 5kHz are applied every 300ms. The voltage

level of the pulse is controlled from 250V to 5kV. Coupling onto

the cable is via a capacitive clamp which is essentially two metal

plates which sandwich the line under test to provide a distrib-

uted coupling capacitance. This clamp is fed from the transient

generator which is constructed with a spark gap driven from an

energy storage capacitor which enables the high voltages to be

generated with the fast rise times. The AD421 is programmed

so that 10mA flows in the loop during the experiments. The

results from this test were as follows:

Burst Amplitude Results

Comments.

500V Positive

PASS

Normal Operation.

500V Negative

PASS

Normal Operation.

1kV Positive

FAILS

Loop current resets to 4mA.

1kV Negative

FAILS

Loop current resets to 4mA.

2kV Positive

FAILS

Loop current resets to 4mA.

2kV Negative

FAILS

Loop current goes to 20mA.

The board achieves class 1 performance for application of a

burst of 500V, ie it continues to work as normal. For all other

bursts the operation can be classified as class 3 where user

intervention is required to restore the part back to normal

operation after the burst has been removed.

Further development work on the AD421 evaluation board is

being planned to see if the above performance can be improved

upon.